why not "est-ce"?
"Est-ce que c'est un oignon?" is correct. In fact, it's more correct than "C'est un oignon?" But you need the whole phrase "est-ce que." You'll need somebody else to tell you why...
"est-ce" would work. i used it.
It worked (30-12-2013). But it doesn't sound nice.
Can I say "C'est-il un oignon?" (Obviously still struggling a bit with the different question forms.)
You can say: "Est-ce un oignon?" if you want to use the verb-inversion form.
Also, "Est-ce que c'est un oignon?" and just "C'est un oignon?" are acceptable, though slightly less formal I believe.
When do you use est-ce que and when do you just use est-ce